The present invention relates in general to the fabrication of double-polysilicon bipolar transistors (i.e. extrinsic base and emitter made of polycrystalline silicon (poly-Si), in contrast to the single-polysilicon bipolar transistors with only the emitter made of poly-Si). More particularly, the present invention relates to a process for fabricating a self-aligned double-polysilicon bipolar transistor having an epitaxially-grown base.
The conventional process for fabricating a double-polysilicon bipolar transistor includes, as shown in FIG. 1a, forming, in or on a semiconductor substrate 1 having a first conductivity type (for example a substrate made of single-crystal silicon having an n-type conductivity), a region 2 having a second conductivity type the opposite of the first (for example a region made of silicon and/or SiGe alloy of p-type conductivity). Thus, a base region 2 of the second conductivity type (p) and a collector region underlying the base region 2 and of the first conductivity type (n) are formed in the substrate 1. The next step includes forming, on the base region 2, a first thick layer 3 made of poly-Si of the second conductivity type (for example, a heavily-doped p+ layer), and, on this thick poly-Si layer 3, a thick layer of a dielectric material 4 such as SiO2 or an SiO2/Si3N4 bilayer.
An emitter window 5 is then formed by conventional photolithographic etching of the dielectric material layer 4 and the thick poly-Si layer 3 down to the base region 2 (FIG. 1b). As shown in FIG. 1c, after depositing a passivation layer 6 on the sidewalls of the window 5, the window 5 is filled and the dielectric material layer 4 is coated with a second poly-Si layer of the first conductivity type (for example heavily doped with n+) to form, after masking and conventional etching, an emitter region 7 made of poly-Si of desired geometry and size. The rest of the first thick p+-doped poly-Si layer forms the extrinsic base region of the transistor, whereas the remaining part of the thin silicon or SiGe alloy layer 2 located under the emitter region 7, forms the intrinsic base region of the bipolar transistor.
The doping of the various layers can be carried out conventionally, either in situ, i.e. during the formation of the layers, or after formation of the layers by ion implantation. Also in a conventional manner, it is possible for the various doped layers to undergo activation annealing of the dopants. Furthermore, the base region 2 may be conventionally formed from a single-crystal SiGe/Si bilayer.
The etching of the window 5 usually comprises a first conventional step of etching the layer of dielectric material 4, for example SiO2, stopping on the first poly-Si layer 3, then a second step which is also conventional etching of the first thick poly-Si layer 3 stopping at the base region 2 (overetching). This overetching may, in practice, result in removing the active base region of the transistor, therefore leading to a defective device.
An object of the invention is to provide a process for fabricating a double-polysilicon bipolar transistor, while avoiding any risk of overetching the active base region of the transistor.
It has been found, according to the invention, that it is possible to avoid any risk of overetching the active base region of a double-polysilicon bipolar transistor during its fabrication using a process comprising: (a) the production of a substrate comprising a collector region of a first conductivity type and a base region of a second conductivity type the opposite of the first; (b) the formation on the base region of an interlayer made of germanium and/or SiGe alloy (preferably polycrystalline) of the second conductivity type; (c) the formation over a predetermined zone of the Ge and/or SiGe alloy interlayer of an etch-stop film; (d) the formation on the interlayer and the etch-stop film of a first thick layer made of polycrystalline silicone (poly-Si) of the second conductivity type; (e) the formation on the first poly-Si layer of an outer layer of a dielectric material; (f) the etching, through an appropriate mask, of a window preform in the layer of dielectric material and the first thick polysilicon layer, stopping on the etch-stop film, then removal in the window preform of the etch-stop film; (g) the selective removal in the window preform of the Ge and/or SiGe alloy layer in order to form an emitter window having a bottom formed by an exposed zone of the base region and of the sidewalls; (h) the formation of a passivation layer on the sidewalls of the emitter window; (i) the formation of a second polysilicon layer of the first conductivity type, so as to fill the emitter window and cover the outer dielectric material layer; and (j) the etching of the second polysilicon layer of the first conductivity type in order to form an emitter region of the desired geometry and size.
The processes of forming Ge and/or SiGe alloy layers are well known and it is possible to use, for example, conventional chemical vapor deposition (CVD) processes. Similarly, polysilicon layers may be formed by any conventional process such as CVD and plasma-enhanced chemical vapor deposition (PECVD). The formation of layers made of dielectric material, for example SiO2 or Si3N4, such as the layer of step (c) and the passivation layer of the sidewalls of the emitter window, is also conventional in bipolar transistor technology.
The Ge and/or SiGe interlayer of step (d) has a thickness which usually varies from 2 to 125 nm and is preferably about 2 to 40 nm. The first polysilicon layer usually has a thickness of 50 to 250 nm, and preferably of 125 nm to 250 nm. The doping of these layers is carried out conventionally, either by in situ doping with a dopant of appropriate conductivity, or, after deposition of the layers, by conventional ion implantation of a dopant of appropriate conductivity. Conventionally, annealing steps may be carried out in order to activate the dopants.
SiGe alloys are well known. Among these alloys, mention may be made of Si1xe2x88x92xGex where alloys 0 less than x less than 1 and Si1xe2x88x92xxe2x88x92yGexCy where alloys 0 less than xxe2x89xa60.95 and 0 less than yxe2x89xa60.05. Preferably, SiGe alloys with a relatively high germanium content, usually 10 to 50% at germanium, will be used, since the SiGe alloy etching selectivity with respect to silicon and to SiO2 increases with the germanium content of the alloy.
The masking and the etching of the various layers to form the emitter window preform may be done by any process, such as a isotropic etching by dry means, for example by plasma. The selective removal of the Ge or the SiGe film may be carried out conventionally via a chemical oxidant, for example with a 40 ml 70% HNO3+20 ml H2O2+5 ml 0.5% HF solution or by isotropic plasma etching. This removal is controlled so as to take off that part of the poly-Ge or poly-SiGe layer which is located at the bottom of the emitter window preform and possibly so as to etch a small fraction of this layer under the first polysilicon layer.
The presence of the etch-stop film, for example made of SiO2, means that the etching of the first polysilicon layer, which is usually by plasma etching, will definitely stop at the stop film. This is because the plasma etching of polysilicon is selective with respect to SiO2 and an end-of-etching detection signal can be used conventionally. So, while the first polysilicon layer is being etched, the interlayer definitely cannot be etched to such an extent that this layer is pierced, with consequent etching of or damage to the base region. The etching of the stop film and consequently of a fraction of the interlayer may then be carried out without risk of damaging the surface of the base region, by using known etching techniques which are gentler and more selective.